When the Ravens open the preseason Thursday night against the Atlanta Falcons, barring a sudden setback, fans can rest assured that they will see Joe Flacco under center.

The identity of that center, however, is not as clear. Rookie Gino Gradkowski and Justin Boren are the leading candidates to fill in for 15-year veteran Matt Birk, who is not expected to play after sitting out the past eight practices due to back spasms.

Although neither player has been with the team longer than two years, Flacco said the chemistry between him and both centers is improving.

"These guys haven't been around for long … but I would say it's as close to as good as it gets for a short amount of time," Flacco said. . "Obviously, we're growing and trying to get the shotgun snaps better and there are things that are going to come up during the year that we may not be perfectly synchronized with yet. That's why we worked a little bit of the silent count [Tuesday] and things like that — things that we haven't done much of in the past. So it's definitely a work in progress, but I can't complain about them."

Birk, who has not missed a regular-season or playoff game in the three years that he has spent with the Ravens, said Gradkowski and Boren are ready to start against the Falcons.

"The young guys are getting a lot of good reps and there's a lot of stuff coming at you in training camp — especially when you're a rookie," Birk said . "It's the proverbial 'drinking through the firehose.' So they're just trying to calm down a bit and if they have any questions, any type of resource I can be, I try to be there."

The preliminary expectation is that Gradkowski will get the nod to replace Birk. The first of two fourth-round picks in April's NFL draft, Gradkowski has been working steadily with Flacco and the rest of the first offensive line in tackles Michael Oher and rookie Kelechi Osemele and guards Marshal Yanda and Bobbie Williams.

"I feel pretty good," the 6-foot-3, 300-pound University of Delaware product said. "I feel good with the offense. You can never be fully prepared for your very first NFL game. I know I won't be 100 percent ready, but I feel ready, and I'm very excited. I know Matt will be right there beside me helping me out."

Gradkowski acknowledged that he might have to battle some nervous jitters when he's standing inside the Georgia Dome. "But that's natural, having some jitters for your first NFL game," he said. "But I don't think it will cause a problem for me out there, especially after you get the first snap over with."

Of the trio, the 6-2, 315-pound Boren — who made the last season's practice squad as an undrafted rookie — may have the most familiarity with the offensive playbook and blocking schemes.

"Yeah, I think I'm ready," he said. "We've been working hard in the offseason. At center, you have to know the offense better than the average position because you're making all the calls. You're pretty much the quarterback of the offensive line. So we've been working hard to do that. We've been rotating at practice, and things have been going great. It's great competition."

But Boren primarily played guard at Ohio State, and his relative inexperience showed on Saturday during the Ravens' practice at M&T Bank Stadium. He misplayed two shotgun snaps to Flacco, sailing one over the head of the 6-foot-6 quarterback and then mistakenly thinking that Flacco was right behind him during another play.

Coach John Harbaugh said both Gradkowski and Boren are developing at a faster pace than expected due to Birk's prolonged absence.

"I think they're progressing very well — probably a little quicker than we expected mentally," Harbaugh said. "Both of those guys are really sharp guys. We've had some issues with gun snaps like you might expect. So we've got to smooth those things out. They both are very athletic, have very good feet, and the question with those guys — because they're not the biggest guys — is, how are they going to hold up physically? We've got a pretty big front, and they've held up fine."

Birk, who plans to watch the game from the sideline, said he will dispense advice whenever his teammates need it.

"They've been working hard — Gino and Justin," Birk said. "Those guys have worked extremely hard this offseason to be ready."